When Lee Berger hired two cavers to explore some South African caves in search of fossils, he probably did not expect to discover a whole new species of hominid.

However, in the Rising Star cave, lying 48km north of Johanessburg, a newfound chamber revealed a huge collection of fossils, enough to identify the eighth Homo species : Homo naledi. This name was given after the Sesotho (South African language) name for the Rising Star cave : Dinaledi.

A total of 1550 fossil fragments belonging to 15 individual skeletons were retrieved from the cave. The skeletons belong to all kind of specimen, male and female, adults and infant. This is the largest collection of hominid fossils ever found in Africa in a single site. Found in an isolated environment, the fossils are remarkably preserved, and almost every bone of the body was found multiple times.

The first excavation began in November 2013. Two years of work later, two papers were in the journal eLife, making Berger’s finding accessible to the public.

A secret chamber

No wonder the previous explorations of the Rising Star cave did not reveal the chamber, now known as the Dinaledi chamber. One of the cavers hired by Berger, Steve Tucker, discovered it by wedging himself in a crevice, and noticing his feet did not touch the bottom. It turns out that this 18cm wide crevice was a 12 meters long shaft opening into a chamber, a cave 70m from the outside world.

Cross section of the Rising Star cave showing the Dinaledi chamber. This image is from the October issue of National Geographic magazine.

When Berger was informed that hominid fossils were found in the cave, he assembled a 60 person team on the site, and recruited new cavers to further explore the chamber, bearing in mind the very narrow access. The thousands of fossils were retrieved by a team of 6 women, and as they did, it soon became obvious to Berger that he was sitting on something monumental, and that his team would not be sufficient.

Berger therefore recruited more people through a call on Facebook. A new team was soon formed, mostly of early-career scientists with various specialties. Depending on their background and previous experiences, the new recruits were assigned a body part. You then got hip people, thorax people, upper-limb people and so on. After a while, each group began communicating with the others, trying to decipher the bigger picture : H. naledi started to reveal its secrets.

Morphology of Homo naledi

naledi appears to be a very peculiar species, displaying traits both from early hominins, such as Australopithecus, and members of the Homo genus. This anatomical mosaic is not new, some other early hominins such as A. sediba already displayed such features. From the finding of Berger’s team, it appears that H. naledi shares three major features with the Homo genus :

– Locomotion : it has long lower limbs, strong M. gluteus maximus (that’s the buttocks muscle, by the way) insertions and humanlike ankle and foot.

– Manipulation : H. naledi shares several aspect in the wrist, thumb and palm morphology with members of the Homo genus. However, the fingers’ bones exhibit a bend that was not previously observed. The ridges in the thumb bones indicate a very powerful grip, so in all likelyhood, this animal was very apt at climbing.

– Mastication : Around 150 teeth were found in the cave. Like other members of the Homo genus, H. naledi had relatively small teeth and did not displayed a powerful mastication like Australopithecus.

Dinaledi skeletal specimens, from Berger’s paper in eLife

On the other hand, the flared hip bone, the shoulders and the ribcage resemble those of Australopithecus. H. naledi was a rather strange animal, a long distance walker – one of the defining traits of the Homo genus – but still a good climber. Would the fossils not have been found in an isolated location, it may have been speculated that the bones came from different species. Berger described it as âA Mr. Potato Head disasterâ!

naledi was an upright hominin, standing around 150cm tall and weighting around 45kg, with a brain volume of around 450 to 550 cubic centimeters, a size comparable to that of Homo erectus, slightly larger than a chimpanzee’s.

A reconstruction of Homo naledi‘s head from bone scans by paleoartist John Gurche This image is from the October issue of National Geographic magazine.

Berger’s team has also uploaded scans and 3-D models of the fragment to an open database, so if you own a 3-D printer, you can print out your very own models of H. naledi fossils ! Scans can be found at this adress.

The best is yet to come

Two big question remain unanswered : how old are these fossils, and why were 15 skeletons in such a remote location ?

Until the datation is carried out, the fossils reveal almost nothing about hominin evolution. For now H. naledi could be an ancestor of the early humans or could have lived alongside modern humans. The bones were encased in clay which preserved them, but also made it difficult to date them from their environment. Since the datation process of the bone themselves requires the destruction of the sample, Berger’s team must first finish the characterization and description of the fossils.

It is speculated that H. naledi would be around 2 million years old as a species. However, the population of the Dinaledi cave might be more recent than that. The datation of the bone will confirm this hypothesis and place H. naledi somewhere between H. habilis and H. erectus, or lead us to rethink what we thought we knew about human evolution, maybe in less linear and more “bush-like” manner.

The question of why these 15 H. naledi were in that cave is also a very complicated one. Several possible explanations were ruled out, and the one that remains would be – to say the least – groundbreaking.

No indication of a direct vertical passageway to the surface was observed, so we can speculate that the access to the cave was always as arduous as it is today.

Occupation : This place was not a settlement of any kind. It is absolutely not practical, and such a large group would have left signs of occupation, such as debris, which are found in every other site where long-term occupation occurred. None of that was found in the Dinaledi cave. There were only H. naledi bones, along with the bones of a few rats and birds.

Water transport : Analysis of the sediments present in the cave rule out the hypothesis of water transport of the bodies into the Dinaledi chamber during inundations.

Predator accumulation : The bodies were not dragged there by some predator, as no signs of carnivore remains, nor the remains of other prey animals were found in the cave. Furthermore, none of the bones presented marks made by teeth, or any sign of violence whatsoever.

Death trap : The scenario in which a group of H. naledi were trapped in the cave and died, during some catastrophic event, does not seem very likely : why would the animals chose to go this deep in the cave, in the dark ? Moreover, the sedimentological analysis shows that the accumulation of the fossils occurred over a long period of time. In other words, not in a single event.

What could explain the presence of 15 skeletons in a remote cave, over an extended period of time, with no signs of damage, no signs of occupation, no trace of any other animal, nothing else ?

Everything points out to a deliberate action from H. naledi. It is very possible that this place is a burial site. Now, burial of the dead has previously been attributed to both Homo sapiens and Neanderthals. The oldest confirmed burial site was found in Israel, in the Tabun Cave, and is around 100 000 years old.

But we are talking here about a burial site that might be 2 million years old. That is something very, very new. Which is why Berger’s team is studying very seriously this possibility, but won’t say “it’s a burial site” until they are quite sure of it.

It is not impossible though. Modern Apes, especially chimpanzees, do have an understanding of death. Mourning behaviors have been observed in Pan troglodytes, and perhaps the reason why moving or burying the bodies has not been observed yet in primates is because most of them don’t stay in a place for long. When living in some kind of settlement, funeral practices make much more sense.

What we are looking at might be the story of a small brained primitive hominid deliberately disposing of its dead in a repeated fashion in an underground chamber. For that, they would have had to crawl in a narrow passage, climb that knife-edge ridge called the Dragon’s Back, then somehow squeeze the body into the 18cm wide opening of 12-meter chute that opens on the chamber, with fire as the only mean of lighting. There is something very human-like in this way of taking risks and deploying such efforts to dispose bodies to a remote area where nothing can get to them. This rather romantic vision has already inspired artists.

Artistâs depiction of Homo naledi disposing of its dead. Art by Jon Foster

More exotic and much less credible hypothesis have been made, such as “the Dinaledi chamber is the first penal system”, making the cave a prison. If I had to make my own crazy guess, I would go for the first religious sacrifices, were selected individuals were put in the cave and left to die in order to appease Mother Earth. Feel free to make up your own crazy theories in the comments !

I guess there are two morals to this story : If you did not find what you were looking for the first time, it might be worth the shot to try again, and if you want to go and dig hominids fossils on remote locations, you’d better start following paleoanthropologists on Facebook !